Sweden has released a photograph of a mysterious vessel seen within its territorial waters as the country’s military continues to hunt for a foreign submarine.

Since Friday ships, helicopters and about 200 troops have been involved in a search following a tip-off.

The photo is being presented as evidence of suspicious foreign underwater activity. The vessel allegedly submerged after the picture was taken.

Rear Admiral Anders Grenstad, Royal Swedish Navy explained what he thought the vessel could be:

“It could be a submarine. It could be a diver using a moped-like underwater vehicle and it could be divers that don’t have any business on our territory – that’s the range of foreign underwater activity we are looking into.”

The search has reawakened memories of the Cold War when Sweden repeatedly sought out suspected Soviet submarines along its coast.

The incident comes amid rising tension with Russia among the Nordic and Baltic states over Moscow’s involvement in Ukraine. Last month Sweden scrambled military jets as Russian fighter bombers strayed into Swedish airspace.